1. Field of the Invention
The invention relates to a method and an apparatus for melting and/or refining, in particular in melting or refining units with cooled walls.
It is an object of the invention to provide a method and an apparatus with reduced energy consumption when melting and/or refining inorganic substances, preferably glasses, glass-ceramics and crystals. The invention is preferably to be used in a melting or refining unit with cooled walls, for example in a skull crucible or a conductively heated tank.
2. Description of Related Art
The prior art has described a range of melting units with cooled walls, in particular skull crucibles, which are used to melt glasses using radiofrequency.
In the present context, a radiofrequency-heated skull crucible is to be understood as meaning a crucible which is formed from cooled metal tubes and is surrounded by a radiofrequency coil. There are gaps between the metal tubes, allowing the radiofrequency energy to be introduced directly into the glass melt.
In the present context, a conductively heated tank or a conductively heated crucible is to be understood as meaning a tank or crucible in which heat is supplied at least in part by direct or alternating currents introduced by means of electrodes.
The melting of glasses in a skull crucible brings a number of advantages, on account of the formation of a skull layer of the same type of material as the glass on the cooled metal tubes. High-purity and/or very aggressive and/or high-melting glasses can be melted down and kept molten in a skull crucible.
High-melting glasses which only melt at temperatures over 1600 to 1700° C. can be melted in a melting unit with cooled walls, since the skull layer of material of the same type as the glass is constantly reformed even at high temperatures, thereby providing protection against external impurities.
Also, when melting chemically highly aggressive glasses, a skull layer of material of the same type as the glass is formed, preventing the glass melt from attacking the cooled metal tubes.
When melting high-purity glasses, the skull layer of the same type of material as the glass in particular also prevents crucible material dissolved by the glass melt from entering the glass melt. Since oxidized constituents of the cooled metal tubes, such as in particular metal ions, can diffuse in very small quantities through the skull layer, if the purity requirements are extremely stringent, it is necessary to use special metal tubes, such as for example metal tubes made from platinum metals, aluminum tubes or plastic-coated metal tubes.
However, melting in a melting unit with cooled walls, such as for example a skull crucible, has the drawback that the method is very energy-consuming, since the walls have to be very strongly cooled in order to cool the walls and/or produce a skull layer of the same type of material as the glass, and consequently a very large amount of energy is withdrawn from the glass melt.
The temperature difference between the heated melt and the cooler wall region is in this respect a direct measure of the energy loss through dissipation of heat. This temperature difference increases at higher melting or refining temperatures, and consequently the energy loss through dissipation of heat is also increased at the same time.
Since both radiofrequency energy and fed-in conductive currents are expensive energy carriers, for example compared to fossil fuels, the abovementioned melting and refining methods have the reputation of being expensive and energy-consuming. In particular an additional increase in temperature, by analogy with conventional tanks, was regarded as a direct measure of additional costs.
With conventional melting tanks, it is attempted to keep the dissipation of heat through the walls and the floor, i.e. the energy loss, as low as possible by using good thermal insulation of the melting tanks.
The energy loss per unit weight EV is dependent on the throughput. Assuming the same grade of glass, an increase in throughput is generally associated with an increase in temperature.
Conventional tanks with walls made from metallic or ceramic refractory materials can only be heated to the temperature which is permissible for the refractory materials; for metallic refractory materials, such as for example platinum metals, no higher than 1500° C., and for ceramic refractory metals no higher than 1600° C., or in exceptional cases up to at most 1700° C. for brief periods of time.
With conventional tanks, therefore, the maximum melting temperature is determined by the refractory materials used. By determining the maximum melting temperature, the refractory materials also predetermine the maximum throughput.
Furthermore, with conventional tanks, there may be problems with introducing the energy required to increase the temperature into the tank, since the higher burner temperature attacks the roof to a greater extent, and in the case of additional electrical heating the electrodes are extensively attacked.
The increase in temperature is also associated with an increase in the convection of the glass melt in the melting tank, which in turn leads to a considerably greater attack on the tank.
An increased attack on the tank firstly shortens the service life of the tank and secondly has an adverse effect on the quality of glass. Therefore, in conventional tanks, an increase in the throughput always entails the risk of a deterioration in the glass quality as a result of melting relics. In the case of a gas-fired tank, a higher glass temperature at the same time also means a higher top furnace temperature. This is associated with increased production of environmentally harmful nitrogen oxides, NOx, in the atmosphere. Many tanks are already running at the currently applicable limits.
Consequently, with conventional tanks there are very tight limits on increasing the throughput by increasing the temperature, and consequently it is not appropriate to do so from an energy perspective.
When melting in a melting unit with cooled walls, such as for example a skull crucible, for the same melting temperature the heat loss per unit area is very much higher than the energy loss per unit area with the well thermally insulated walls of a conventional tank.
Furthermore, the energy loss as the temperature increases rises to a greater extent with a skull crucible than with conventional tanks, as explained above. This significantly higher energy loss from the melting units with cooled walls compared to the conventional tanks is the reason for melting in units with cooled walls, such as for example melting in skull crucibles, being relatively uncommon.
Despite having considerable advantages over conventional tanks, the skull melting method has hitherto only been used where conventional melting methods have failed.